Arrangement for RF power delivery to a gas discharge laser with cascaded transmission line sections

ABSTRACT

RF power is transmitted to a CO 2  gas discharge laser form a source of RF power via a series combination of transmission line sections. The lengths and characteristic impedances of the transmission line sections are selected to transform the impedance of the RF power source to the operating impedance of the laser.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to radio frequency (RF) powersupplies. The invention relates in particular to RF power supplies fordriving carbon dioxide (CO₂) slab lasers.

DISCUSSION OF BACKGROUND ART

FIG. 1A is a block diagram schematically illustrating a prior-artarrangement 10 for driving a CO₂ (gas discharge) slab laser. Poweramplifier 12 represents an output power amplifier stage of an RF powersupply for driving the laser. The output impedance of such a poweramplifier is typically 50 Ohms. The amplifier output is typically atfrequencies up to about 150 megahertz (MHz) or greater. Such anamplifier stage can include a number of parallel combined transistoramplifier modules. The amplifier output is connected to electrodes (notshown) of a CO₂ laser 18 via a 50 Ohm coaxial transmission cable (line)14 and an impedance matching network 16. The transmission line can beconsidered as having an input port or node 13 and an exit port (or node)15.

Cable 14 can be of any length and, accordingly, the power supply can belocated remote from the laser. The impedance at entrance port 13 of thetransmission line is essentially the same as an exit port 15 of thetransmission line and is the same as the load impedance of the RFamplifier. The impedance of a CO₂ laser scales inversely in relation tothe power output of the laser. Accordingly as power output increases,the impedance of the amplifier must be matched to increasingly lowerimpedance at the laser, and the size, weight and cost of the impedancematching network increases accordingly.

Recent slab laser designs have included an RF power supply mounteddirectly on the laser such that a long transmission cable is notrequired. FIG. 1B schematically illustrates a prior-art arrangement 20applicable with a directly mounted power supply. Here the RF powersupply is connected by a transmission cable 22 having a length ofone-quarter wavelength at a frequency typically between about 80 MHz and100 MHz, directly to the CO₂ laser. That length would be about 45centimeters (cm) for a frequency of 100 MHz. In this example of FIG. 1Bit is assumed that laser has an impedance of 4 Ohms which is about theimpedance for a laser having an output between about 800 W and 1000 W.Transmission line 22 is assumed to have an impedance equal to the squareroot of the product 50×4 Ohms, i.e., about 14 Ohms. The impedance atentrance port 21 of the transmission line is 50 Ohms and the impedanceat exit port 23 is 4 Ohms. In an arrangement such as the arrangement ofFIG. 1B, if coaxial cable having a desired characteristic impedance isnot commercially available, it will usually be possible to find cablehaving a sufficiently close match that a relatively simple inexpensiveimpedance matching network can be used to match the difference.

A disadvantage of the arrangement of FIG. 1B is that quarter-wavetransmission line 22 is a resonant structure at a single frequency. ACO₂ slab laser has a resonant frequency when the discharge of the laseris not lit (laser not operating) that is higher than the resonantfrequency once the discharge is lit and the laser is lasing (operating).The operating resonant frequency can be as much as 25% or more lowerthan the non-operating frequency for a 1 KW laser. This makes itdifficult, if not impossible, with the arrangement of FIG. 1B to applythe same power efficiently to the laser in both the un-lit and litconditions.

Another disadvantage of the arrangement of FIG. 1B is that low impedancecables, for example cable having an impedance of less than 25 Ohms, aredifficult to obtain commercially without a special order. Cables havingan impedance of 50 or 75 Ohms are readily available and relativelyinexpensive by comparison.

As laser power increases, and accordingly as RF power delivered to thelaser increases (RF power is typically about ten-times greater thanlaser output power), both of the above described arrangements willencounter a problem with heating of the single transmission line alongwhich the power is delivered. This will be true whether the transmissionline is coaxial, a micro-strip line, or any other form of transmissionline. This problem has been solved in the prior-art by an arrangementsimilar to that depicted in FIG. 2.

Here, a CO₂ laser arrangement 24 includes a plurality of poweramplifiers 12 (three in this example) delivering RF power via acorresponding plurality of 50-Ohm transmission lines 14. The outputconductors of the lines are connected together and connected via animpedance matching network 16 to a CO₂ laser 18. The three transmissionlines can have the same (as illustrated) or different lengths. Theimpedance at each entrance port 13 is 50 Ohms and there is an impedanceof 16.6 Ohms at a common output port 17 which is connected to theimpedance matching network. This reduces demands on the design of theimpedance matching network.

A problem with the arrangement of FIG. 2, encountered when driving a 1kW laser, was that in order to effectively light the laser discharge itwas necessary to make the transmission lines one half-wavelength long.This resulted in a bandwidth limitation similar to that described abovewith reference to FIG. 1B, which meant that power could not be deliveredwith maximum efficiency at both the un-lit and lit dischargefrequencies. There is a need for an RF-power delivery arrangement for aCO₂ laser than preferably does not require an impedance matchingnetwork, and has a sufficiently broad bandwidth that power can bedelivered efficiently at both the un-lit and lit discharge frequenciesof the laser.

Additional information about RF power supplies for gas lasers can befound in commonly owned U.S. Patent Publication 2008/0204134, thedisclosure of which is incorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention is directed to apparatus for coupling RF power toa CO₂ gas discharge laser. In one aspect the inventive apparatuscomprises a gas discharge laser having an operating impedance in anoperating condition at an operating resonant frequency when a gasdischarge in the laser is lit. At least one source of radio frequency(RF) power is provided for powering the gas discharge. The RF powersource has a source impedance, which is greater than the operatingimpedance of the laser. At least one plurality of transmission linesections is connected in series in an electrically conductive pathbetween the RF power source and the laser. At least first and secondones of the transmission line sections have respectively first andsecond different characteristic impedances. The length andcharacteristic impedance of each of the transmission line sections isselected to transform the source impedance to a third impedance lowerthan the source impedance for optimizing RF power transmission from theRF power source to the gas discharge laser.

In preferred embodiments of the invention the transmission line lengthsand characteristic impedances are selected such that RF powertransmission is optimized over a frequency range broad enough to includethe operating resonant frequency and the non-operating (unlit discharge)resonant frequency of the laser. Arrangements with a bandwidth of afrequency octave or greater are described. All embodiments can beconfigured such that an L-C matching network is not required.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain principles of the presentinvention.

FIG. 1A schematically illustrates one prior-art arrangement fordelivering RF power from an RF Amplifier to a CO₂ laser via a singlelength of transmission line of arbitrary length and an L-C impedancematching network.

FIG. 1B schematically illustrates another prior-art arrangement fordelivering RF power from an RF Amplifier to a CO₂ laser via a singlelength of transmission line of having a length of one quarter-wavelengthat the frequency of the RF power, without the L-C impedance matchingnetwork.

FIG. 2 schematically illustrates a prior-art arrangement for deliveringRF power from three RF amplifiers via three transmission lines ofarbitrary length and one L-C impedance matching network to one CO₂laser.

FIG. 3 schematically illustrates one preferred embodiment of apparatusin accordance with the present invention for delivering power from an RFamplifier to a CO₂ laser via a series arrangement of two transmissionline-sections each thereof having a length of one-quarter wavelength fora predetermined operating frequency and having different characteristicimpedances between the output impedance of the RF amplifier and alit-discharge impedance of the CO₂ laser.

FIG. 4 schematically illustrates another preferred embodiment ofapparatus in accordance with the present invention for delivering powerfrom three RF amplifiers to a CO₂ laser via three arrangements similarto the arrangement of FIG. 3.

FIG. 5 is a graph schematically illustrating calculated powertransmission as a function of frequency for one example of thetransmission-line section arrangement of FIG. 3.

FIG. 6 is a graph schematically illustrating calculated powertransmission as a function of frequency for one example of thearrangement of FIG. 4.

FIG. 7 is a graph schematically illustrating calculated powertransmission as a function of frequency for one example of the prior-artarrangement of FIG. 2.

FIG. 8 schematically illustrates yet another preferred embodiment ofapparatus in accordance with the present invention for delivering powerfrom four RF amplifiers to a CO₂ laser via a corresponding fourquarter-wavelength-long transmission-line sections having a firstcharacteristic impedance connected in pairs to twoquarter-wavelength-long transmission-line sections having a secondcharacteristic impedance different from the first characteristicimpedance.

FIG. 8A schematically illustrates an additional embodiment of apparatusin accordance with the present invention for delivering power from fourRF amplifiers to a CO₂ laser, similar to the apparatus of FIG. 8 butwherein there is an L-C matching network connected between the secondcharacteristic transmission line sections and the CO₂ laser.

FIG. 9 schematically illustrates yet another preferred embodiment ofapparatus in accordance with the present invention for delivering powerfrom an RF amplifier to a CO₂ laser via a series arrangement of fourtransmission line-sections each thereof having a length of one-eighthwavelength for a predetermined operating frequency and having differentcharacteristic impedances between the output impedance of the RFamplifier and a lit-discharge impedance of the CO₂ laser.

FIG. 10 is a graph schematically illustrating calculated powertransmission as a function of frequency for one example of thetransmission-line section arrangement of FIG. 9.

FIG. 11 schematically illustrates still another preferred embodiment ofapparatus in accordance with the present invention for delivering powerfrom an RF amplifier to a CO₂ laser via a series arrangement of threetransmission line-sections each thereof having a length of abouttwo-thirds of a quarter-wavelength for a predetermined operatingfrequency and having different characteristic impedances between theoutput impedance of the RF amplifier and a lit-discharge impedance ofthe CO₂ laser.

FIG. 12 is a graph schematically illustrating calculated relative powertransmission as a function of frequency for one example of thetransmission-line section arrangement of FIG. 11.

FIG. 13 schematically illustrates a further preferred embodiment ofapparatus in accordance with the present invention for delivering powerfrom an RF amplifier to a CO₂ laser via a series arrangement of threetransmission line-sections each thereof having a different length withtwo thereof having a first impedance between the output impedance of theRF amplifier and a lit-discharge impedance of the CO₂ laser and onethereof having a second impedance higher than both the output impedanceof the RF amplifier and the lit-discharge impedance of the CO₂ laser.

FIG. 14 is a graph schematically illustrating calculated relative powertransmission as a function of frequency for one example of thetransmission-line section arrangement of FIG. 13.

FIG. 15 schematically illustrates another further preferred embodimentof apparatus in accordance with the present invention for deliveringpower from an RF amplifier to a CO₂ laser via a series arrangement oftwo transmission-line sections one thereof having a length of aboutthree quarter-wavelengths at a predetermined frequency and the otherhaving a length of about one-quarter wavelength at the predeterminedfrequency with the transmission line sections having differentcharacteristic impedances between the output impedance of the RFamplifier and a lit-discharge impedance of the CO₂ laser.

FIG. 16 is graph schematically illustrating calculated relative powertransmission as a function of frequency for one example of thetransmission-line section arrangement of FIG. 15.

FIG. 17 is a graph schematically illustrates yet another furtherpreferred embodiment of apparatus in accordance with the presentinvention for delivering power from an RF amplifier to a CO₂ lasersimilar to the arrangement of FIG. 3, but wherein one of thequarter-wavelength-long transmission line sections is replaced by aparallel pair of quarter wavelength long transmission-line sections.

DETAILED DESCRIPTION OF THE INVENTION

Continuing with reference to the drawings, wherein like components aredesignated by like reference numerals, FIG. 3 schematically illustratesone preferred embodiment 30 of an RF-power delivery arrangement inaccordance with the present invention. In arrangement 30 the output of asingle power amplifier stage 12 having an output impedance of 50 Ohms istransmitted by a first transmission line section 32 having acharacteristic impedance of 26 Ohms in series with a second transmissionline 34 having a characteristic impedance of 7.5 Ohms, to match a 4-Ohmoperating impedance of laser 18. By way of example the amplifier stagecan have an output power of about 1.5 kW and operate at a frequency ofabout 100 MHz.

Transmission lines 32 and 34 each have a transmission length ofone-quarter wavelength at a frequency about mid-way between the litdischarge (operating) frequency and the unlit (non-operating) dischargeresonant frequency. It should be noted here that the transmission linesection length, as recited here and in the appended claims refers to theeffective electrical or RF transmission length of the section. Theactual physical length will be shorter to an extent determined by thedielectric constant and the thickness of the insulating materialseparating conductors of the transmission line section. The impedance atport 31 is 50 Ohms. This is transformed to an “image” impedance of 14Ohms at port 33 (the exit port of transmission line 32 and the entranceport of transmission line 34). This image impedance is transformed toanother image impedance of 4 Ohms at exit port 35 of transmission line,matching the impedance of the CO₂ laser.

Transmission line 32 is preferably a flexible coaxial transmissioncable. Such a cable is commercially available from Weico Wire and Cable,Inc. of Edgewood, N.Y. Transmission line 34 is a 7.5 Ohm transmissionline. Such a transmission line may not be available as an“off-the-shelf” item but it can be commercially purchased by specialorder, for example from Micro-COAX Inc. of Pottstown, Pa. While depictedin coaxial form in FIG. 3, for convenience of illustration, it ispreferred that transmission line 34 is fabricated as a quarter-wave,micro-strip transmission line printed on an alumina substrate.Transmission line 32 can also be constructed in this form. Other formsof transmission lines such as strip-lines or parallel pair lines may beused in this and any other embodiment of the invention described hereinwithout departing from the sprit and scope of the invention. Theconstruction of different forms of transmission line is well known bythose skilled in the RF art and a description thereof is not requiredfor understanding principles of the present invention. Accordingly, nosuch description is presented herein.

FIG. 5 is a graph schematically illustrating the calculated powertransmission as a function of frequency for the example of FIG. 3,wherein the quarter-wave transmission-lines 32 and 34 are one-quarterwavelength long for a frequency of 100 MHz. The 0.15 dB bandwidth isabout ±20 MHz. This is sufficient bandwidth to accommodate the abovediscussed 25% difference between un-lit discharge and lit dischargeresonant frequencies. By way of comparison, calculated powertransmission as a function of frequency for the prior-art example ofFIG. 1B is schematically depicted in FIG. 7. This has a calculated 0.15dB Bandwidth of only ±8 MHz which is less than 25% of the centerfrequency of 100 MHz.

FIG. 4 schematically depicts another preferred embodiment 40 of anRF-power delivery arrangement in accordance with the present invention.In arrangement 40, the output of three power amplifier stages 12, eachthereof having an output impedance of 50 Ohms, is transmitted by threecorresponding pairs 32 and 34 of transmission line sections each havingthe lengths and impedance values of transmission line sections 32 and 34of FIG. 3 to a CO2 laser 18A having an input impedance of 1.3 Ohms. Theoutput of the three transmission line sections 34 is connected to acommon node 42 at which the impedance is 1.3 Ohms, i.e., is theimpedance of the parallel combination of three 4-Ohm output impedancesof the three transmission-line section pairs.

The calculated power transmission as a function of frequency ofarrangement 40 is depicted in FIG. 6. It can be seen that this issubstantially the same as for arrangement 30 of FIG. 3 with a 0.15 dBbandwidth of ≅20 MHz

It should be noted here that while three transmission line sectionseries are parallel connected in the arrangement 30, this should not beconsidered limiting and is just a specific example of a general, N-way,combination, where N is two or more. Further, in arrangement 30 thecombination of transmissions line sections 34 could be arranged as aclassical Wilkinson N-way combiner with “balancing” resistors in a staror delta connection with nodes 33. Such balancing resistors can improveport-to-port isolation should that be required because of phase ormagnitude imbalance between power amplifiers 12.

FIG. 8 schematically illustrates yet another preferred embodiment 50 ofa power delivery arrangement in accordance with the present invention.In this example, a Wilkinson N-Way combination with balancing resistorsis used in a binary form, with a balance resistor to provide isolationbetween pairs of amplifiers. In arrangement 50 there are four RFamplifiers 12, the combined power of which is delivered to a CO₂ laser18B having a lit-discharge impedance of 2 Ohms.

Each RF amplifier 12 is connected to a quarter-wave length 32B oftransmission line sections, here, having an impedance of 35 Ohms. Atinput port 31B of the transmission line sections the impedance is theoutput impedance of the RF amplifiers, here, assumed to be 50 Ohms foreach amplifier. The outputs of pairs of transmission line sections 32Bare connected together such that at a common output port 33B theimpedance is 12 Ohms, which is the value of two 24-Ohm individualimpedances in parallel. Ports 33B are connected by an isolating(balancing) resistor R having a resistance equal to twice the 12-Ohmimpedances at ports 33B. The two common ports 33B function as inputports to a corresponding two quarter-wave lengths of transmission line34B each thereof having an impedance of 7 Ohms connected to a commonoutput port 35B. The impedance at the common output is 2 Ohms which isthe value of two 4-Ohm individual impedances in parallel, and matchesthe 2-Ohm (lit discharge) impedance of the laser.

It can be seen that in the electrical-conduction path between any RFamplifier 12 and laser 18B there is only one transmission line length32B and only one transmission line length 33B. The transmission as afunction of frequency can be expected to be similar to that (depicted inFIG. 5) of arrangement 30 of FIG. 3 as it is determined primarily by thelength and impedance of the transmission line sections series-connectedin the conduction path. Those skilled in the art may select othercombinations of RF amplifiers and transmission lines which provide thiscondition without departing from the spirit and scope of the presentinvention. By way of example, there may be six RF amplifiers 12 andlines 32B connected in groups of three to two transmission lines 34B. Inany combination, the use of a balancing resistor or resistors, such asbalancing resistor R of FIG. 8, is optional.

FIG. 8A depicts an additional embodiment 50A of a power-deliveryarrangement in accordance with the present invention. The arrangement of50A is similar to arrangement 50 of FIG. 8 with an exception that laser18B of arrangement 50 is replaced by a laser 18A having an impedance of1.3 Ohms. A conventional L-C impedance matching network 39 is connectedbetween transmission line sections 34B of the arrangement to transformthe 2-Ohm image impedance at node 35B to the 1.3 Ohm operating impedanceof the laser. Preferably the L-C matching network is designed to have abandwidth comparable to that of the transmission line arrangement.

FIG. 9 schematically illustrates still another preferred embodiment 60of a power delivery arrangement in accordance with the presentinvention. This arrangement delivers power from a single RF amplifier12, here, having an output impedance of 50 Ohms (at port 61) to a CO₂laser 18 (at port 69) having a lit-discharge impedance of 4 Ohms. Thepower is delivered via a series chain of four eighth-wave lengths(sections) of transmission line 62, 64, 66, and 68 having characteristicimpedances of 37 Ohms, 20 Ohms, 10.3 Ohms, and 5.4 Ohms, respectively.At intermediate ports 63, 65, and 67, the impedances are 26.6 Ohms, 14Ohms, and 7.5 Ohms, respectively.

FIG. 10 is a graph schematically illustrating calculated transmission(in decibels) for the arrangement of FIG. 9, with impedance values asnoted, assuming a center frequency (the frequency at which thetransmission line sections are one-eighth of a wavelength long) of 70MHz. It can be seen that the 0.15 dB bandwidth extends from this centerfrequency to a frequency of about 200 MHz, which is a bandwidth ofalmost two octaves.

FIG. 11 schematically illustrates still yet another preferred embodiment71 of a power delivery arrangement in accordance with the presentinvention. This arrangement again delivers power from a single RFamplifier 12 having an output impedance of 50 Ohms (at port 71) to a CO₂laser 18 (at port 77) having a lit-discharge impedance of 4 Ohms. Thepower is delivered via a series-chain of three sections of transmissionline 72, 74, and 76 each having a length of about, but not exactly,one-sixth of a wavelength, and having characteristic impedances of 32.82Ohms, 14.14 Ohms, and 6.093 Ohms, respectively. At intermediate ports73, and 75, the image impedances are 21.54 Ohms, and 9.28 Ohms,respectively. Note that the line impedances are rounded in FIG. 11 forconvenience of illustration. The line lengths are varied slightly fromthe one-sixth wavelength to provide a flatter in-band spectral responseand are preferably 0.681, 0.674, and 0.681 quarter-wavelengthsrespectively.

FIG. 12 is a graph schematically illustrating calculated relativetransmission for the arrangement of FIG. 11, with impedance values andline-lengths as noted above, assuming a center frequency of 100 MHz. Itcan be seen that the bandwidth extends from this center frequency to afrequency of about 200 MHz, which is a bandwidth of about one octaves.

It should be noted that with arrangements of the type exemplified inFIGS. 9 and 11, the center frequency for specifying transmission lengthis preferably selected to correspond to the lit discharge frequency orsome somewhat-lower frequency. The bandwidth will be more thansufficient to accommodate the unlit discharge frequency.

In the above-described inventive arrangements exemplary impedance valuesfor transmission lines are selected for a particularly preferredperformance, as lines having specific impedances can be commerciallyobtained on special order as noted above. Special order lines, ofcourse, can be expected to be more expensive than stock lines to anextent depending on the quantity required. In the above-describedarrangements, the particularly preferred values of line-impedance arebetween the output impedance of the RF amplifier and the operatingimpedance of the laser. In the above-described arrangements the totallength of transmission lines is about one-half of a wavelength at anassumed center frequency. Set forth below is a description of howline-length can be used as a variable to accommodate at least onetransmission line section that has an impedance value available fromstock and can even have a value higher than the output impedance of theamplifier.

FIG. 13 schematically illustrates a further preferred embodiment 80 of apower delivery arrangement in accordance with the present invention.This arrangement, once again, delivers power from a single RF amplifier12 having an output impedance of 50 Ohms (at port 81) to a CO₂ laser 18(at port 87) having a lit-discharge impedance of 4 Ohms. The power isdelivered via a series-chain of three sections of transmission line 82,84, and 86, having different lengths. Section 82 has a length of 0.1527of a quarter-wavelength; section 84 has a length of 0.1985 of aquarter-wavelength; and section 86 has a length of 1.0639quarter-wavelengths. The total length of transmission line sections isabout three-eights of a wavelength. The center frequency forestablishing the line-length wavelength is preferably about mid-waybetween the lit discharge frequency and the unlit discharge frequency.

The characteristic impedances of sections 82, 84, and 86 are 7.5 Ohms,75.0 Ohms (higher than the amplifier output impedance), and 7.5 Ohms,respectively. Coaxial cable having an impedance of 75.0 Ohms is readilyavailable commercially. The image impedance values at ports 83 and 85are not calculated and knowledge thereof is not necessary forreproducing this example of the invention.

FIG. 14 is a graph schematically illustrating relative transmission forthe arrangement of FIG. 13 (bold solid curve), with impedance values andline-lengths as noted above, assuming a center frequency of 100 MHz. Thespectral response of the above described example of FIG. 3 is depictedby a fine dashed curve. The 0.12 dB bandwidth of arrangement 80 iscomparable to that of arrangement 30 of FIG. 3.

FIG. 15 schematically illustrates a another further preferred embodiment90 of a power delivery arrangement in accordance with the presentinvention, configured for delivering power from a single RF amplifier 12having an output impedance of 50 Ohms (at port 91) to a CO₂ laser 18 (atport 95) having a lit-discharge impedance of 4 Ohms, as in otherabove-described arrangements. The power is delivered via a series chainof two sections of transmission line 92 and 94. Section 92 has a lengthof 2.95 quarter-wavelengths and section 94 has a length of 0.98 of aquarter-wavelength. Here the total length of transmission lines is abouttwo half-wavelengths. The wavelength here preferably corresponds to acenter-frequency about mid-way between the lit-discharge frequency andthe unlit-discharge frequency.

To alleviate a need for specific values of impedance for the sectionsthe arrangement is configured to provide peak transmission at and abouttwo particular frequencies, here, 87.5 MHz and 112.5 MHz, without regardto transmission in between these frequencies. These frequencies areexamples of lit and unlit-discharge frequencies of a laser such as laser18. Sections 92 and 94 have characteristic impedances of 27 Ohms and10.5 Ohms respectively. These values are very close to those of cablesTC-24 and T-12, respectively available from stock from RF Power SystemsInc, of Peoria, Ariz. The lengths are for a nominal center frequency of100 MHz. The image impedance at port 93 is a purely resistive 26.4 Ohms,as indicated, at the center frequency and is reactive at 21±8j at thedesired peak transmission frequencies.

FIG. 16 is a graph schematically illustrating calculated relativetransmission for the arrangement of FIG. 15 (bold solid curve), withimpedance values and line-lengths as noted above, assuming a centerfrequency of 100 MHz. The spectral response of the above describedexample of FIG. 3 is depicted by a fine dashed curve. It can be seenthat at the particular peak-transmission frequencies the actualtransmission of arrangement 90 is slightly greater than for the“broad-band” example of FIG. 3.

FIG. 17 schematically illustrates yet another further embodiment 100 ofan RF power delivery arrangement in accordance with the presentinvention. Arrangement 100 is similar to arrangement 30 of FIG. 3 withan exception that 7.5-Ohm quarter-wave transmission line section 34 ofarrangement 30 is replaced in arrangement 100 by a parallel pair oftransmission line sections 102 each thereof having a length ofone-quarter wavelength at the nominal center frequency and having animpedance of 15 Ohms to provide the 7.5 Ohms result from the parallelpair.

Those skilled in the art to which the present invention pertains willrecognize that in any of the above described embodiments a singletransmission line section may be replaced with two or more parallelsections, each thereof having the length of the section being replaced.The sections preferably have the same characteristic impedance, but canhave different impedances. This provides a means, albeit with somecomplication, of providing the equivalent of a line-section having adesired characteristic impedance from line-sections having impedancescommercially available from manufacturers' stock. Those skilled in theart will also recognize that while the arrangements of FIG. 9, FIG. 11,FIG. 13, and FIG. 15 are described in a context of delivering power to aCO₂ laser from only one amplifier, these arrangements may be included inpower combining arrangements including a plurality of RF amplifiers ofthe type exemplified in FIG. 4 and FIG. 8. It is also possible toinclude in certain of the inventive arrangements an L-C impedancematching network between the last transmission line section in thearrangement and the CO₂ laser as exemplified in FIG. 8A. The L-Cmatching network, however, should have a bandwidth comparable to thebandwidth of the transmission-line arrangement to avoid mitigating thebroad-bandwidth advantage of the inventive transmission linearrangements. It is emphasized, however, that the impedance matching(transformation) flexibility of the present invention described aboveshould be sufficient that an L-C network is not required for optimizingpower transmission to the laser in most arrangements.

In summary, the present invention is described above in terms of anumber of preferred embodiments. The invention is not limited, however,to the embodiments described and depicted. Rather, the invention islimited only by the claims appended hereto.

1. Apparatus, comprising: a gas discharge laser having an operatingimpedance in an operating condition at an operating resonant frequencywhen a gas discharge in the laser is lit; at least one source of radiofrequency (RF) power for powering the gas discharge, the RF power sourcehaving a source impedance, the source impedance being greater than theoperating impedance; at least one plurality of transmission linesections connected in series in an electrically conductive path betweenthe RF power source and the laser, at least first and second ones of thetransmission line sections having respectively first and seconddifferent characteristic impedances; and wherein the length andcharacteristic impedance of each of the transmission line sections isselected to transform the source impedance to a third impedance lowerthan the source impedance for optimizing RF power transmission from theRF power source to the gas discharge laser.
 2. The apparatus of claim 1,wherein there is only one RF power source and only one plurality oftransmission line sections, and wherein the plurality of transmissionline sections is connected directly to the gas discharge laser and thethird impedance is about equal to the operating impedance of the gasdischarge laser.
 3. The apparatus of claim 1, further including an L-Cimpedance matching network having an input impedance on an input sidethereof and an output impedance at an output side thereof, and whereinthere is only one RF power source and only one plurality of transmissionline sections, the plurality of transmission line sections is connectedto the input side of the L-C matching network, the gas laser isconnected to the output side of the L-C matching network, the thirdimpedance is about equal to the input impedance and of the L-C matchingnetwork, and the output impedance of the L-C matching network is aboutequal to the operating impedance of the gas discharge laser.
 4. Theapparatus of claim 1, wherein there are N RF power sources and acorresponding N pluralities of transmission line sections.
 5. Theapparatus of claim 4, wherein the pluralities of transmission linesections are connected in parallel directly to the laser and the thirdimpedance is about N-times the operating impedance of the laser suchthat the parallel combination of third impedances is about equal to theoperating impedance of the laser.
 6. The apparatus of claim 1, whereinthe laser has a non-operating resonant frequency in a condition wherethe gas discharge is not lit, the non-operating resonant frequency beinglower than the operating resonant frequency, and wherein the lengths andcharacteristic impedance of the transmission line sections are selectedsuch that RF power to the laser is optimized over a frequency rangeincluding the non-operating resonant frequency and the operatingresonant frequency.
 7. The apparatus of claim 6, wherein there are onlythe first and second transmission line sections in the pluralitythereof, and wherein each of the transmission line sections iselectrically about one-quarter wavelength long at a wavelengthcorresponding to a frequency between the operating resonant frequency ofthe laser and the non-operating resonant frequency of the laser.
 8. Theapparatus of claim 6, wherein there are first, second, and thirdtransmission line sections in the plurality thereof, and wherein each ofthe transmission line sections is electrically about one-sixthwavelength long at a wavelength corresponding to a frequency about equalto or less than the operating frequency of the laser.
 9. The apparatusof claim 6, wherein there are first, second, and third transmission linesections in the plurality thereof, and wherein the transmission linesections have a combined electrical length of about three-eighths of awavelength at a wavelength corresponding to a frequency between theoperating resonant frequency of the laser and the non-operating resonantfrequency of the laser.
 10. The apparatus of claim 9, wherein the secondtransmission line section has an impedance higher than the sourceimpedance.
 11. The apparatus of claim 9, wherein there are first,second, third, and fourth transmission line sections in the pluralitythereof, and wherein each of the transmission line sections iselectrically about one-eighth wavelength long at a wavelengthcorresponding to a frequency about equal to or less than the operatingfrequency of the laser.
 12. The apparatus of claim 1, wherein the laserhas a non-operating resonant frequency in a condition where the gasdischarge is not lit, the non-operating resonant frequency being lowerthan the operating resonant frequency, and wherein the lengths andcharacteristic impedance of the transmission line sections are selectedsuch that RF power to the laser is maximized at both the non-operatingresonant frequency and the operating resonant frequency.
 13. Theapparatus of claim 12, wherein there are only first and secondtransmission line sections in the plurality thereof, the first andsecond transmission line sections having electrical lengths of aboutthree-quarters of a wavelength and one-quarter of a wavelength,respectively, at a wavelength corresponding to a frequency between theoperating resonant frequency of the laser and the non-operating resonantfrequency of the laser.
 14. The apparatus of claim 13, wherein the firstimpedance is lower than the source impedance and higher than the secondimpedance and the second impedance is higher than the operatingimpedance of the laser.
 15. The apparatus of claim 1, wherein any of thetransmission line sections in the plurality thereof is connected in aparallel combination with at least one other transmission line sectionto provide the effect of a single transmission line section having animpedance intermediate the impedances of the transmission line sectionsin the combination.